An input signal generated by collecting speech with a microphone may include a noise component, or a signal component corresponding to voice of a speaker may be small in the input signal. When an input signal includes a noise component or when the signal component is small, speech of a speaker may be unclear in the input signal. In addition, in the case of a device configured to recognize speech of a speaker in an input signal and perform processing corresponding to the speech, if the speech of the speaker is unclear, the device may fail to perform desired processing due to deterioration of the accuracy of speech recognition. To address this, a technology called Auto Gain Control (AGC) that automatically adjusts the level of an input signal has been utilized (see Japanese Laid-open Patent Publication No. 56-84013, for example).
However, excessive adjustment of the level of an input signal may increase distortion of the input signal or may even enhance a noise component, and speech of a speaker may not typically become clear. In particular, when one word is long, the voice of a speaker tends to become smaller as the speech comes close to the ending of the word. As a result, a signal corresponding to the word may not be clearly identified in the input signal. In such a case, even if the conventional AGC is applied to the input signal, the speech of the speaker included in that input signal may remain unclear.
Hence, as one aspect, an object of the specification is to provide a speech enhancement device capable of making clear speech of a speaker which is included in an input signal, even when volume of speech produced by the speaker changes according to a time from beginning of speech production.